An organic light-emitting diode (OLED) is an active light-emitting device in which light is emitted by recombination of electrons and holes and a phosphor is excited. An organic light-emitting display including the organic light-emitting diode can be used in wall mounted or portable displays owing to its fast response speed, low direct-current driving voltage, and ultra thinness, in comparison to a passive light-emitting device that uses a separate light source such as a liquid crystal display.
The organic light-emitting diode embodies a color using pixels in which red, green, and blue sub pixels are provided. In a method for driving the subpixels, the organic light-emitting diode can be classified as a passive matrix organic light-emitting diode (PMOLED), or an active matrix organic light-emitting diode (AMOLED) employing a driving method using a thin film transistor (TFT).
With respect to AMOLEDs, the thin film transistor has non-uniform device characteristics due to aspects of the TFT manufacturing process. For example, a polysilicon thin film transistor (p-si TFT) manufactured using an excimer laser to crystallize the silicon has non-uniform device characteristics that cause the power output to be unstable, i.e. the output current of the TFT varies even though the same data voltage provided to the TFT.
To compensate for the non-uniform characteristics, several driving methods have been suggested. These driving methods include a current driving method, a voltage driving method, and a digital driving method.
FIG. 1 is an equivalent circuit diagram illustrating a conventional current driving active matrix organic light-emitting device.
The conventional active matrix organic light-emitting device 10 that compensates for the non-uniformity of the thin film transistor comprises first to fourth thin film transistors (T1 to T4), a storage capacitor (Cst), and an organic light-emitting diode (OLED). The first to fourth thin film transistors (T1 to T4) comprise P-channel metal oxide semiconductor field effect transistors (MOSFET), and polysilicon thin film transistors (p-si TFT).
The organic light-emitting diode (OLED) emits light corresponding to the magnitude of the applied signal current (IEL).
The first thin film transistor (T1) is connected between a source voltage (VDD) and the organic light-emitting diode (OLED), and supplies the signal current (IEL) to the organic light-emitting diode (OLED).
The storage capacitor (Cst) is connected between the source voltage (VDD) and a gate of the first thin film transistor (T1) and stores the data voltage.
The second thin film transistor (T2) is connected between a gate and a drain of the first thin film transistor (T1), and has a gate connected to a first scan line. During the time period when the first scan signal is being applied to the second thin film transistor (T2) through the first scan line, the gate and the drain of the first thin film transistor (T1) becomes a common node, which allows the second thin film transistor to drive the first thin thin film transistor (T1).
The third thin film transistor (T3) is connected between the first thin film transistor (T1) and a current source (I), and has a gate connected to the first scan line. The third thin film transistor (T3) is in an ON state when the first scan signal is applied through the first scan line. This provides a current path for an output current (I) of the current source to store the storage capacitor (Cst) with a data voltage proportional to the output current (I).
The fourth thin film transistor (T4) is connected between the first thin film transistor (T1) and the organic light-emitting diode (OLED), and has a gate connected to a second scan line. The fourth thin film transistor is in an ON state when the second scan signal is applied through the second scan line so that a current is supplied to the organic light-emitting diode (OLED), thereby driving the organic light-emitting diode (OLED). During this time, the first scan signal is such that the second and the third thin film transistors (T2 and T3) are in OFF state.
Though the second and third thin film transistors (T2 and T3) are in OFF state, the data voltage proportional to the output current is stored in the storage capacitor (Cst), the first thin film transistor (T1) is driven by the data voltage, thereby supplying the signal current (IEL) to the organic light-emitting diode (OLED).
In the conventional organic light-emitting diode (OLED), since the output current can be driven irrespective of the variations in the thin film transistor characteristics, non-uniformity in the luminance resulting from these variations can be compensated for.
In the conventional current compensation for organic light-emitting device 10, when a low gray level picture is displayed, the output current (I) is very low. The storage capacitor and the data line load are charged because the data line load acts as a parasitic capacitor on the data line, thereby removing current to adequately drive an OLED. Accordingly, the ability of an OLED to express a low gray level picture is diminished significantly. The deterioration of the capability to express a low gray level picture increases in seriousness in a large sized area where the data line load increases.
Thus, there is a need to provide an organic light-emitting device and organic light-emitting display that compensates for the variations in the thin film transistor characteristics, thereby making the luminance uniform, while preventing the deterioration of the ability of the OLED to express low gray levels.